The present invention relates to a two-component type dry developer, particularly to a two-component type dry developer most suitable as a developer for a printer and a copying apparatus using an electrophotographic process.
An electrophotographic process is frequently utilized in a printer and a copying apparatus (hereinafter referred to as the "printer"). Typically, an electrostatic latent image is first formed on a photosensitive medium such as a photosensitive drum and the image is subsequently made visible by exposure to toner during development. Although the toner produces a visible image on the photosensitive medium, the toner image is typically transferred and fixed to another suitable medium such as paper.
The two-component type dry developer (hereinafter referred to as the "developer") consists of a mixture of a toner and a carrier. Typically, the toner becomes fixed to the photosensitive medium during development due to the electrostatic force of attraction between the photosensitive medium and the oppositely charged toner particles. Development of the latent image is possible only if the toner particles are adequately charged. Toner is charged as a result of sufficient frictional contact with the carrier. However, the toner is relatively soft due to its high resin content and the carrier, consisting of iron powder, is rather hard. Thus, the surfaces of the carrier gradually becomes contaminated with toner during the frictional process. This developer deterioration due to contamination of the carrier surfaces by toner poses serious problems, particularly in high speed printers where the carrier and toner strike each other violently.
Conventional developers used in high speed printers generally consist of a mixture of a toner containing resin as its principal component and a magnetic carrier consisting of nearly perfectly spherical iron powder particles. The use of spherical carrier material in the conventional developer reduces the severity of the impact caused by the collision between the toner and carrier and thus contamination of the carrier surfaces is retarded. However, such conventional developers have the following drawbacks.
The spherical carriers mentioned above have fewer points for contact than other types of carriers, such as needle, rod or plate-like carriers. Therefore, the resistivity of the developer is fairly high making it difficult for a electric current to flow. As a result, the tone has a tendency to adhere only to the boundaries of image planes. This phenomenon is often referred to as the "edge effect". Occurrence of the edge effect not only results in obscureness of the image but also makes it virtually impossible to reproduce the original image density and mutual spacing in the printed image plane. The image density and mutual spacing characteristics frequently provide important information, bar code symbols being a prime example.
A further disadvantage of the above-mentioned conventional developer is the difficulty in controling the concentration of the toner. Optimally, a developer should contain only enough toner to produce a single-layer on the surfaces of individual carriers. Unfortunately, the use of spherical carriers results in an extremely sharp distribution of the specific surface area and it thus becomes difficult to control the supply of toner to be used during the development by the toner. Optimization of toner concentration for conventional developers using spherical carriers is difficult.
Furthermore, the conventional developer has the additional disadvantage of having a weak magnetic force because of its spherical form. The weakness of the magnetic force tends to cause the developer to disperse from the surface of the magnetized roll used for the transport following toner development. Therefore, image quality is degraded due to the poor development as a result of the collisions between the dispersed carriers and the surface of the electrostatic latent image-forming medium. These collisions disfigure the surface of the photosensitive medium. This phenomenon is increasingly apparent in high speed printers in which the magnetized rolls rotate at high speeds. Although this problem, in fact, can be solved by enlarging the carrier particle diameter and strengthening the magnetic force, such an approach also aggravates the edge effect.
On the other hand, in developers in which needle, rod and plate-like carrier are used, the carriers have many contact points compared to the spherical carrier. This feature produces a print with a good black image. Good results are obtained because the inherent low resistivity of these structures allows current to flow easily. Since the geometry of the carrier allows a wide range of toner distribution along its surface area, it is easy to control the optimal value of the toner concentration. In addition, there is virtually no problem of the developer dispersing from the surface of the magetized roll since the carrier has a strong magnetic force when used in a high speed printer. However, contamination of the carrier surfaces by the toner is significantly increased and progresses rapidly due to the sharp corners of this type of conventional carriers. In this way, the life of the developer is shortened and the brush-in magnetic "brushes" become stiff due to the strong magnetic force. This causes the magnetized roll to bear an excessive load, making it impossible in some instances to achieve the high speed development.